This invention relates to a sintered Fe-Cr-Co type magnetic alloy having high density and improved magnetic properties and to a method for the production of an article made of it.
An Fe-Cr-Co type magnetic alloy containing some additives has been developed which is workable through hot- and cold-working and which is comparable to ALNICO-5 (trade name) with respect to magnetic properties. The magnetic material of this type has been widely used so far due to its improved properties.
There are many methods of producing an article made of a magnetic material on an industrial scale, for example, rolling, casting and sintering. It is impossible, however, to apply rolling to ALNICO- and ferrite-type magnetic alloys, but it is advantageous to produce the Fe-Cr-Co type magnetic alloy articles through rolling, since the magnetic alloy of this type is ductile. Therefore, many alloy compositions and a variety of manufacturing processes therefor have been proposed regarding the Fe-Cr-Co type magnetic alloy. However, usually, rolling is not desirable for mass-production of a magnetic alloy and rolling is expensive, because a series of complicated steps, i.e. forging, rolling, annealing, etc. are necessary. Thus, in general, the application of rolling is limited to the production of special shapes, e.g. plate-, or wire-shaped products. In addition, it has been thought necessary to use a variety of additives to prevent the oxidation and nitriding of chromium during melting and heat treatment in the process of manufacturing the alloy.
Casting may in general be applied to a material which is hard and brittle like the ALNICO type alloy. However, since the Fe-Cr-Co type material is ductile, it is sometimes difficult to remove castings from a runner and casting defects, such as sand inclusion and misrun, are inevitable. In addition, operational efficiency is not satisfactory. Furthermore, since a variety of additives have to be added to prevent the oxidation and nitriding of chromium during melting, casting is not attractive from an economical point of view.
On the other hand, with sintering there are no such problems as in the rolling or casting method. Sintering is suitable for the mass-production of Fe-Cr-Co type magnets on an industrial scale. However, as is well known in the art sintering has disadvantages in that the density and magnetic properties of sintered products are not satisfactory. Japanese Patent Laid-Open Specification Nos. 54-33205 and 53-43006 show that sintering density is improved by the addition of B, Si, C, etc. However, in these cases, the resulting magnetic properties are not satisfactory; (BH).sub.max is not greater than 5.0 MGO. In addition, the cobalt content is more than 20%; therefore, the product is expensive.